This invention relates to erasable optical media and in particular to erase techniques for erasable optical media. Optical media store information; computer data or audio/video information for example, in digital form. The most common optical media formats include computer hard disc peripheral memory, video laser disc, compact disc and digital tape.
The information stored on an optical medium is represented in binary form by physically creating a pattern of bumps in the medium. The bump pattern can be read by focusing a laser beam on the medium and reading the reflection. The bumps, or pits, contained in the medium increase the optical path of the beam, creating an interference pattern that decreases the intensity of light reflected from the area containing the bump. The read device detects these variations in light intensity and converts this data to an electrical signal which is then processed into the desired output.
To create the bump pattern on the medium, a write beam is passed over the medium and turned on at those locations where a bump is desired. Several different erasable optical media structures exist, and the formation of the bump by the write beam differs slightly in each of the specific structures. One example of a dye polymer erasable optical disc medium is that described in U.S. Pat. No. 4,874,709 to Clark et al. which is incorporated herein by reference for all purposes. The medium described in Clark contains both an expansion layer and a retention layer. The expansion layer absorbs energy emitted by the write beam causing it to heat up and expand into the retention layer which has also been softened by the energy of the write beam. Once the write beam passes over, the media starts to cool. The retention layer, however, returns to its original hardened state much faster than the expansion layer can contract. The region of the expansion layer which expanded into the retention layer continues to cool but is now held in tension by the hardened retention layer and a bump is formed.
To erase the bump, an erase beam, usually having a different wavelength from the write beam, is passed over the bump location. The erase beam is focused on the plane of the retention layer and causes the retention layer to become soft. The erase beam does not, however, cause the expansion layer to expand. When the retention layer becomes soft, the cooled expansion layer is no longer held in tension by the retention layer and snaps back to its original position, thereby eliminating the bump.
Typical erasable systems are dual beam systems in which the erase beam passes over the medium in advance of the write beam and performs a bit by bit, or bump by bump, erasure of the data stored on the disc, returning the media to its virgin state. The write beam then imprints a new pattern of bumps on the medium to represent the new data. A distance of three to four tracks separates the center of the erase beam from the center of the write beam to prevent newly written data from being inadvertently erased. The read beam passes over the medium at a later time to retrieve the data currently stored on the medium.
The bit by bit erasure system imposes severe requirements on the erase beam. In particular, the narrow focus erase beam of the typical system must remain in close alignment with the bump to be erased to ensure complete erasure. This requirement leaves the erase system susceptible to alignment, mechanical vibration and media warpage problems.
The narrow focus erase beam also means that the optical medium must receive a short concentrated dose of energy to produce the desired thermal change in the medium. When the erase beam is turned off, the retention layer is quenched by the relatively cool surrounding material. The quenching process happens so rapidly that the retention layer cannot respond fast enough to the cooling process and occasionally a small mark is locked into the media. This mark constitutes an incomplete erasure which can be read by the playback system as a data bump. This particular problem becomes even more acute in optical media which use a single layer to store data instead of the multilayer system of Clark.